Premature atrial contractions (PACs) occur when a site in the atria other than the sinus node develops automaticity of a rate greater than that of the sinus node. Such a site is called an ectopic focus. An ectopic focus can usurp control of the atria from the sinus node for one or a few consecutive beats causing a single PAC or a “salvo” of PACs. If the ectopic focus remains in control for a long period, this is referred to as a focal atrial tachycardia.
Some conventional pacemakers have an ability to respond to PACs. For example, some conventional pacemakers treat PACs that occur outside of the post-ventricular atrial refractory blanking period (PVARP) as sinus P-waves and respond by tracking PACs or, in non-tracking modes, by inhibiting output of atrial pacing stimuli. Further, if such a conventional pacemaker has enabled an atrial fibrillation (AF) suppression algorithm (e.g., dynamic atrial overdrive), PACs that occur outside of the PVARP typically trigger an increase in atrial overdrive pacing rate (e.g., in a manner similar to sinus P-waves that occur outside of the PVARP).
AF suppression algorithms generally request an increase atrial overdrive pacing rate only if a predetermined number of P-waves occur within a predetermined number of cycles. For example, a conventional AF suppression algorithm will request an increase in atrial overdrive pacing rate if two P-waves occur in 16 consecutive pacing cycles. In addition, the requested rate increase is typically based on a present pacing rate (e.g., 110% of the present rate).
After a programmable number of overdrive cycles at the increased rate, the AF suppression algorithm requests a gradual decrease or decay in overdrive rate, for example, until a request for another increase, or until the overdrive rate reaches the higher of a base rate and a sensor indicated rate. If request calls for a rate increase that increases the overdrive rate above a maximum allowed overdrive rate, then the overdrive rate is clipped to the maximum allowed overdrive rate.
Aforementioned algorithms, however, may respond inappropriately to repeated salvos of PACs. For example, repeated salvos of PACs can result in ever increasing rate requests that drive the overdrive rate up to the maximum allowed overdrive rate (e.g., typically equal to the maximum sensor rate in conventional pacemakers) and, once reached, request that the maximum rate is maintained for an extended period of time. Physiologic consequences of maintaining a high overdrive rate for an extended period of time are manifold and may include pain, annoyance, heightened catecholaminergic response (e.g., positive feedback) and even tachycardia-induced heart failure. Thus, while it may be appropriate for an algorithm to increase the overdrive pacing rate in response to PACs, because overdrive pacing may act to suppress PACs, conventional AF suppression algorithms typically do not produce the best outcome in patients that experience salvos of PACs. In essence, if PACs persist or are persistent, then extended overdrive pacing at a maximum rate may do little, if nothing, to improve cardiac function and, at worst, it may lead to a critical condition.
Overall, a need exists for algorithms that respond appropriately to PACs and/or other atrial events. Various exemplary methods, devices, systems, etc., are described below which may address aforementioned needs and/or other needs.